1. Technical Field
This invention relates in general to the detection of coating imperfections on a coated web and, more particularly, to a system and method for recognizing predefined-types of coating imperfections in a web through the acquisition of optical density variation information, for example, from a moving, continuous web substantially uniformly, transmissively illuminated.
2. Background Art
Research and development efforts in the photographic materials and paper materials industries often focus on various types of imperfections in a moving coated web. These imperfections may, for example, result from disturbances in the coating process, such as may occur during the sensitization of photographic film. Research and development efforts attempt to isolate, through process modeling, the source of an on-going disturbance-type in a coating process. Coating imperfections of particular interest to the industries are continuous-type imperfections and point-type imperfections. These imperfection types, which can occur in one or more coating levels on a support web, are typically indicative of a disturbance or design related problem in the coating process.
An effective on-line imperfection recognition system and method would enable one to discern, characterize and confirm various models of the coating process, thereby determining the disturbance causing such an imperfection. Two significant issues, however, must be addressed by any imperfection recognition system before adequate optical data can be collected from sensitized coatings under examination. First, the system must be able to extract small density changes from the obtainable spatial and temporal noise background. Secondly, the system must provide adequate illumination within the spectral bandwidth of the usable contrast range, while avoiding solarization of any sensitized web.
State-of-the-art efforts to quantize moving web disturbances have most commonly been implemented as laser scanning systems. For example, continuous laser beams are often swept by multifaceted polygon mirror scanners across moving webs of film or paper support, and focused with dedicated optics onto a discrete detector such as a photomultiplier tube. Various detector configurations enable data acquisition in either a reflective or transmissive mode. Unfortunately, such laser scanner packages can be expensive and typically have limited anomaly detection capabilities.
Specifically, such laser scanning packages are almost universally unable to process data associated with very narrow lines and streaks which may be imbedded in the signal noise background. (Also, laser scan output processing packages, in general, remain less sophisticated than those accompanying state-of-the-art imaging technologies such as solid state cameras.) Therefore, a need is recognized to exist today in the photographic and paper materials industry for a more effective and less expensive technique to extract and characterize imperfections from background data including inherent noise variations, and particularly low-level, narrow continuous-type imperfections in a moving coated web.